Ultrastructural evidence for vertical transmission of SARS-CoV-2

Sir,

Since the initial correspondence by Goldsmith et al. (2020) in The Lancet, numerous electron micrographs of putative severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) virions in biopsy and autopsy tissues have been published. A recent review of these images (Bullock et al., 2021) indicated that previous ultrastructural reports of virions in placental tissue were misidentifications. Placental histology of mothers and neonates, both testing positive for SARS-CoV-2, is typified by chronic histiocytic intervillositis and trophoblast necrosis, with RNA in-situ hybridization/immunohistochemical findings localizing viral RNA/viral antigens to the syncytiotrophoblast (Schwartz and Morotti, 2020). This histopathology is considered as a risk factor for vertical transmission of SARS-CoV-2, which has been documented infrequently to date (Bukowska-Ośko et al., 2021).

Ten days prior to a caesarean section performed at 30 weeks of gestation due to decreased fetal movements, a mother tested positive for SARS-CoV-2 on routine polymerase chain reaction (PCR) assay. Two days after delivery, the neonate tested positive for the virus by rectal swab. Histology of the formalin-fixed placental tissue showed high-grade lymphohistiocytic villitis with extensive histiocytic (CD68-positive) intervillositis, with massive perivillous fibrin deposition (+/-60% of the placental parenchyma), low-grade fetal vascular malperfusion and diffuse villous oedema (Figure 1 ). RNA was extracted from a section of formalin-fixed placental tissue using the Promega Maxwell 16 System. The extract tested positive for SARS-CoV-2 using the Applied Biosystems TaqPath COVID-19 CE-IVD RT-PCR assay. The cycle threshold values obtained for each gene were N=28.65, S=29.25 and Orf1=27.45. Transmission electron microscopy of the formalin-fixed tissue revealed the presence of coronavirus particles in membranous vacuoles within the syncytiotrophoblast, thus providing ultrastructural evidence of vertical transmission.

Figure 1.

Placental tissue infected with severe acute respiratory syndrome coronavirus-2. The cytoplasm of the multi-nucleate placental syncytiotrophoblast was found to contain numerous vesicles filled with virions. These virions are typical of coronaviruses, clustered within membrane-bound vesicles derived from the endomembrane system, and in the electron-dense nucelocapsids appearing in section as dots within the virions (insets: circled enlargement of a virion with arrows pointing to nucleocapsid cross-sections). The virions are generally spherical, with a maximum measured diameter of 127 nm. The dotted line demarcates the trophoblast from the stroma. N, nucleus. Scale bars (insets) = 0.1 µm.